Small fit. The Birkenstock Madrid is the oldest member of the model family and a true classic. Launched half a century ago as a "gymnastics sandal," this simple one-strap model has become a staple in fashion. The upper material is made from the skin-friendly, durable synthetic material Birko-Flor®. Birko-Flor is made from easy-care and hygienic PVC, which is lined on the inside with a soft, breathable fleece layer. This high-quality material is very gentle on the skin, comfortable, and also very durable.